The present invention relates to an amplifier circuit having an excellent NF characteristic in a high frequency range.
In the field of radio communication systems, a high-power signal is often received while a low-power signal is received. In particular, in portable receivers and vehicle-mounted receivers such as a keyless entry system or a cellular phone, the intensity of a signal received by an antenna is very weak. Thus, in order to avoid an influence on an intensity variation of the received signal, a low noise amplifier (LNA) for amplifying the weak signal is provided to a receiving system.
When such a receiver includes an integrated circuit (IC), various special design restrictions have to be taken in circuit designs, such as prevention of a DC current variation in a current amplification factor (hFE) of the radio frequency (RF) devices due to the manufacture instability of a transistor, which affects major RF characteristics, such as the gain noise factor of an LNA. That is, an amplifier circuit should be stable in an output current of the collector even hFE of the RF device is unstable. Therefore, in order to obtain an output current proportional to the input current, a current mirror circuit including a passive element for preventing the leakage of an RF signal is used in a bias circuit (e.g., see Non-Patent Document 1).
FIG. 11 is a circuit diagram illustrating an LNA as a related art. The LNA shown in FIG. 11 generally includes a transistor Q1 of which the emitter is connected to the ground and a bias circuit 10. The transistor Q1 amplifies an input signal from an input terminal 11 connected to the base of the transistor Q1 via a coupling capacitor C1. The output is got as an amplified signal from an output terminal 13 connected to the collector of the transistor Q1 through a coupling capacitor C2. A coil L is connected to the collector of the transistor Q1 in parallel to the transistor Q1.
The bias circuit 10 includes a transistor Q2, a transistor Q3, and a resistor Re. The transistor Q1 and the transistor Q2 form a current mirror circuit. The bases of the transistor Q1 and the transistor Q2 are connected to the emitter of the transistor Q3. The emitter of the transistor Q2 is connected to the ground through the resistor Re. The resistor Re is provided to increase the input impedance of the bias circuit 10 greater than the input impedance of the transistor Q1.
The collector of the transistor Q2 and the base of the transistor Q3 are connected to each other, and are supplied with a current from a constant current source 21. The voltage from a voltage source Vcc2 is applied to the collector of the transistor Q3. A capacitor Cf for filtering a noise is connected to the collector of the transistor Q2 and the base of the transistor Q3.
A function of the bias circuit can be implemented by the configuration in which the collector and the base of the transistor Q1 are connected via a resistor. However, the collector current of the transistor Q1 does not change even if the above-mentioned bias circuit 10 is unstable in the current amplification factor (hFE) of the transistor Q1 due to the manufacture instability. In other words, the LNA having the bias circuit including a current mirror circuit allows obtaining more stable output current characteristic than that of the LNA having the bias circuit including only a resistor.
[Non-Patent Document 1] “Technology for Semiconductor Circuit Design”, Nikkei Business Publications, Inc., Apr. 4, 1987, p. 233-275, written by FUJITA Yasuhiro, SUMI Tatsumi, KATSUYAMA Takashi and WAKAI Shuzo, supervised by TAMAI Tokumichi.
In the aforementioned LNA, when a high frequency signal is input from the input terminal 11, a high-frequency signal is leaked to the base of the transistor Q2 or the emitter of the transistor Q3. The high-frequency signal leaked to the emitter of the transistor Q3 affects the transistor Q3, even though the high-frequency signal leaked to the base of the transistor Q2 is filtered by the capacitor Cf. In addition, the noises from the voltage source Vcc1 or the constant current source 21 affect the transistors Q1 through Q3.
As a result, the noise figures (NF) of the LNA in the high frequency area is deteriorated, that is, an NF is increased. Hence, the LNA having an excellent NF characteristic even in a high-frequency area is desired. In a power-up circuit disposed in a transmission system as well as the LNA, equally, there is a need for a power-up circuit having an excellent NF characteristic even in a high frequency area. It is desirable that the output current characteristic is stably maintained.